John Felde | University of Maryland | USA

First Double Chooz Neutrino Oscillation Result

Today the Double Chooz collaboration presented our first results with about 100 days of single detector data at the LowNu11 conference at Seoul National University, South Korea. The presentation was given by our spokesperson, Herve de Kerret, and was such an exciting moment for our entire collaboration. A press release was submitted to interactions.org.

In case a refresher is necessary, our experiment searches for the last unmeasured mixing angle, θ13, in the three-neutrino mixing matrix, via the disappearance of νe produced by the dual 4.27 GWth Chooz B reactors. This simply means that if the neutrinos are in fact oscillating, we should measure fewer neutrinos at our detector than what we would have expected the reactors to produce. The formula for this is given by:

where we see that sin22θ13 sets the amplitude of the oscillation.

As I stated before, our initial analysis was performed on about 100 days of far detector only data. This graph shows both the data taking efficiency broken up into sub categories, and the total data taking time. During the August to September period, myself and others were on site to conduct the first radioactive source calibrations of the detector. The special data acquired during this time helped understand the detector response and was used to determine certain errors on our measurement.

By combing though the data we can identify neutrino interactions by their unique signature. The detection reaction is called inverse beta decay, and results in the neutrino creating a positron (the anti-particle of the electron) and a neutron. The detector can measure the positron’s energy, and the energy released when the neutron captures on a Gadolinium nucleus inside the liquid scintillator. The double signal is beneficial for reducing random backgrounds since the mean time between the events is about 30 micro seconds.

Counting up all of the candidate neutrino events, one can compare the number of detected neutrinos to the number expected based on the total power output of the nuclear power plant, information which is provided to us by the power company. The following plot shows our detected neutrino rate per day along with the expected rate (blue dashed line), the average rate is 42.6 + 0.7 neutrinos per day. We see great evidence that our extracted neutrino candidates are in fact directly correlated to the reactor power, as they should be.

Neutrino Candidate Rates (no background subtraction)

In the detection interaction, the positron will carry away most of the parent neutrino energy. Since the neutrino energy is relevant for the oscillation probability, studying the “prompt” (positron) energy spectrum shape yields information on the value of θ13. Below is the prompt energy spectrum obtained along with the best fit:

Prompt energy spectrum from 100 days of far detector only.

The data, black dots, are shown as the number of neutrino events for each prompt energy bin. In blue is the expected distribution if there were no oscillations, and in red is our best fit distribution based on the data. In green, pink, and blue are the major background distributions. These backgrounds must be accounted for, and contribute to the systematic error on our best fit value. The rate + shape analysis gives a best fit value of:

sin22θ13 = 0.085 + 0.029(stat) + 0.042(syst)

By itself, our result is not earth shattering, after all it is consistent with zero, but this is the first θ13 sensitive reactor neutrino measurement since the original Chooz result over ten years ago! Both the statistical and systematic uncertainties will improve as we include more data in the analysis, and with the construction of the near detector, our systematic errors will be greatly reduced. This plot shows the projected sensitivity of Double Chooz into the future:

Projected sensitivity with single (solid) and dual (dashed) detectors

As my first experience with preparing a physics result, the whole process has been enlightening. The fervor of our daily (and very early for California) meetings over the past few weeks has certainly demonstrated the deep passion of our collaborators to this experiment and this physics. We look forward to more data and a better understanding of neutrino oscillations.